hello !! i'm an A level spanish student, so i don't have a teacher's perspective but i will say that you should be fine with the latin american accent, one of my friends was taught GCSE spanish by a latin american teacher and she uses elements of a latam accent whens she speaks.
hopefully a teacher can reply to this thread, or if it's possible for you to ask any teachers directly who teach A level about the grammar aspect. I guess it just depends on the differences between the grammar (which i'm unaware of what exactly the differences are but im sure they're enough if you're worried about it) but I'm pretty sure most places teach and examine castellano, unfortunately.
good luck for if you do decide to take spanish A level !!!!